Olympus 38DL PLUS, User Manual

Page: 87 / 318

DMTA-10004-01EN, Rev. D, November 2016
Basic Operation 75
— Sound attenuation or absorption:
In many organic materials such as low density plastics and rubbers, sound
energy is attenuated very rapidly at the frequencies used for ultrasonic
gaging. This attenuation typically increases with temperature. The maximum
thickness that can be measured in these materials will often be limited by
attenuation.
— Velocity variations:
An ultrasonic thickness measurement is accurate only to the degree that
material sound velocity is consistent with the instrument calibration. Some
materials exhibit significant variations in sound velocity from point to point.
This happens in certain cast metals due to the changes in grain structure that
result from varied cooling rates, and the anisotropy of sound velocity with
respect to grain structure. Fiberglass can show localized velocity variations
due to changes in the resin/fiber ratio. Many plastics and rubbers show a
rapid change in sound velocity with temperature, requiring that velocity
calibration be performed at the temperature where measurements are made.
Phase reversal or phase distortion
The phase or polarity of a returning echo is determined by the relative acoustic
impedances (density × velocity) of the boundary materials. The 38DL PLUS
assumes the customary situation where the test piece is backed by air or a liquid,
both of which have a lower acoustic impedance than metals, ceramics, or plastics.
However, in some specialized cases, such as measurement of glass or plastic liners
over metal, or copper cladding over steel, this impedance relationship is reversed
and the echo appears phase reversed. In these cases, it is necessary to change the
appropriate echo detection polarity in order to maintain accuracy (see “Detection
of Echo 1 and Echo 2” on page 235). A more complex situation can occur in
anisotropic or inhomogeneous materials such as coarse-grain metal castings or
certain composites, where material conditions result in the existence of multiple
sound paths within the beam area. In these cases, phase distortion can create an
echo that is neither cleanly positive nor negative. Careful experimentation with
reference standards is necessary in these cases to determine effects on
measurement accuracy.
5.3 Measuring Thicknesses
You can start measuring thicknesses once a transducer is connected (see “Setting Up
the Transducer” on page 63) and once the instrument is calibrated (see “Calibrating
the Instrument” on page 67).